Quality Control and the Art of Error ProofingFebruary 22, 2008 By: Brian Schriver
The elimination or—at the very least—the maximum reduction of product defects is paramount to manufacturers' efforts to maintain customer satisfaction. Error proofing is a common sense approach that uses awareness, detection, and prevention to meet this goal. Because sensors are a fundamental part of this quality-control process, you need to know their strengths and weaknesses so that you can match the right sensing technology with your application.
An Ounce of Prevention
Error proofing is the implementation of mechanisms to prevent product defects. Based on the philosophy that even the smallest number of defects is unacceptable, error proofing maintains that the best way to eliminate defects is to prevent them from happening in the first place.
There are three keys to stopping the proliferation of defects:
- Awareness, which involves training, audio-visual aids, and general assistance for personnel to prevent mistakes
- Detection, which is promoted through the introduction of manual or automated inspection techniques to filter out defects
- Prevention, which includes process improvements or automation to ensure no errors are (or can be) made
Error proofing drastically reduces defects by preventing errors and by catching those that have been made as soon as possible. Examples of these two processes can range from color-coding, which helps the system distinguish between visually similar components, to sensing that determines if an automotive suspension assembly is moved to the next operation before a bolt is placed.
The Sensor's Role
Sensors play a key role in error proofing, and while most are used as simple detection devices, a few are designed to support preventative measures. Sensors used to detect defects come in all shapes, sizes, and technologies, from standard inductive proximity and photoelectric sensors to specialty sensors designed to prevent defects.
Sensors are used primarily to determine the presence or positioning of a part or a feature on a part. Limit switches are cost-effective and easy to apply, but the fact that their operation requires contact with the part prohibits their use in many applications. Inductive sensors are generally the most economical noncontact solution, assuming the parts to be detected are metallic and the sensor can be mounted close enough to accommodate the relatively short sensing ranges (<40 mm) associated with this technology. They're also rugged, making them a good all-around error-proofing solution in harsh industrial environments.
For applications requiring greater sensing distances and/or detection of nonmetallic parts, photoelectric sensors are often the best—and simplest—solution. Infrared sensors in standard sensing modes (e.g., diffuse, retroreflective, and transmitted beam), as well as more specialized sensors in background/foreground suppression, wide-angle, and fixed-focus modes, address the lion's share of error-proofing applications. Laser-based models are also available for sensing extremely small parts and part features at a relatively greater distance.
Specialized photoelectric sensors designed with error-proofing processes in mind address those situations where standard sensing packages are not suitable or are difficult to apply. Photoelectric light arrays using 2D scanning technology can create a light screen to sense an object regardless of its orientation. This is ideal for the detection of parts being ejected from a machine and a host of other applications focused on early-detection error proofing.
The same scanning technology is also used in parts-verification arrays (PVA) that are specifically geared toward error prevention, notably in bin-picking applications. Spanning the sensing fields of multiple PVAs across assembly station bins and wiring them to a controller programmed with the necessary logic can achieve a virtually error-free bin-picking process. Job lights on the sensors show the assembler the correct picking sequence. In the event the assembler attempts to pick an incorrect part, a warning light indicates the error. Additional fault enunciation can be achieved via controller logic in conjunction with a tower light or audible alarm.
Sensors are an integral part of the quality-control process. There's no shortage of sensing technologies to effectively address all your error-proofing applications, whether it's simple presence sensing or bin-pick error-prevention specialty sensors. A complete and successful error-proofing effort will most likely involve a mix of these sensing technologies, as well as audio-visual aids, awareness training for personnel, and some creative thinking in the development of new preventative processes. All are key to driving not only product quality but also customer satisfaction. Manufacturing errors become product defects only if they make it out the door—and into your customer's hands.
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